Over at Dispatches from the Culture Wars, there's a post about the old anti-abortion argument that goes, "What if the fetus you're about to abort was going to cure cancer?" The logical response, as Ed points out, is, "What if that fetus was going to be a mass murderer or rapist?" Which, of course, is at least as likely and demonstrates the flawed logic of this argument.
In fact, I think this argument actually leads us in exactly the other direction: I argue that this line of thought actually would encourage abortions. How? Let's see.
Along the lines of the "what if he or she was going to cure cancer?" I would argue that rarely, if ever, is a good discovery made by a single person. That is to say that quite often new and groundbreaking ideas are in the zeitgeist at the time they are discovered and thus often there is parallel development amongst different people (or teams, in modern science). For instance, if we were to lose Newton, we wouldn't have lost calculus, because Leibnitz had the same idea independently at the same time. If we lost Darwin, we don't lose evolution, because Wallace had the same idea. We lose Einstein, we don't lose relativity, because others were working on the equations as well. There are few breakthroughs that depend solely on one person and would not likely have come about regardless.
However, there is a strong body of evidence, not indisputable, but strong, that a lot of bad things are very dependent upon a single individual. In Holocaust studies, there is an axiom that goes, "No Hitler, no Holocaust." This is because, while there was a nationalist, anti-Semitic atmosphere in post-WWII Germany, the proponents of such sentiments were broken up into various groups who spent their energy fighting amongst each other over small points of ideology rather than banding together to gain power. No one else on the stage of incipient German nationalism was able to get these various groups to come together, and when Hitler did so, the so-called Fuhrer cult quickly developed, whereby adherents were loyal to the person of Hitler rather than any particular point of Nazi belief or policy. It can be (and has been) argued that no one other than Hitler could have formed the Nazi Party and seized power over Germany as he did. In addition, relating to the Holocaust specifically, while most of the right-wing German nationalists at the time were, to some degree, anti-Semitic, almost no one other than Hitler actually intended to wipe the Jews out, and even virulent anti-Semites in the early Nazi movement assumed Hitler was being dramatic when he talked about extirpating the Jews. Even if some other right-wing German nationalist had been able to do what Hitler did and unite the factions, that person would likely not have led Germany to the Holocaust as Hitler did.
All this is speculative, of course, and there are counter-arguments to the so-called "Great Man" theory of history. But still, the "Great Man" theory of history is a valid, well-researched theory that at least we have to consider in light of the "What if this fetus was going to cure cancer?" argument. We have a great deal of evidence that indicates that few, if any, great discoveries or advances in science or human endeavor or contingent upon one person, while a great deal of evidence exists that for many of the great evils in the world, one "Great Man" is necessary.
As such, it would not be unreasonable to argue that, since any given fetus is less likely to be personally responsible for a world-altering beneficial discovery or event than a horrible one, it is logical to abort that fetus to avoid that chance. In fact, taken to its logical conclusion, the "What if he or she was going to cure cancer?" argument indicates that we should, in fact, abort all fetuses, since each one is more likely a Hitler than an Einstein. Which shows, I think, the logical vacuity of the anti-abortion argument in the first place.